Generally, the procedure for molding a radial tire involves manufacturing a green tire, i.e., a tire before to be subjected to a curing process, which consists of first and the second steps.
The first step is that an inner layer, a carcass layer, a bead, an apex layer and a sidewall layer are assembled on a cylindrical drum to form the first case as shown in FIG. 1. The second step is that said case manufactured by said first step is shaped to the same type as nearly completed product, and a breaker layer and a tread layer are adhered, in turn, to said carcass layer as shown in FIG. 2(a).
Then, as shown in FIG. 2(b), said breaker layer and tread layer are adhered completely by pressing of a stiching roller thereto to complete forming of shape of a green case. Said tread being used in the second step is obtained by extruding a shape as shown in FIG. 3, by a conventional extrusion process, and then cutting the resulting product with a knife. Said tread is cut in bias manner as shown in FIG. 4, in order to maximize an area of end contact when the tread is adhered along the circumference of a circle in the second step of the molding procedure.
However, after the extrusion molding, both ends of the tread cut by the knife cannot readily be placed at a bias angle sufficient to extend the contact area of both ends of the tread.
Furthermore, in the conventional radial tire, the proportional weight of the tread is also very high, wherein it has a range of 30.about.40% per total tire weight, and thus it exerts an important effect on uniformity and balance of the final tire.
A method for manufacturing the conventional tread will be described with reference to the accompanying drawings.
Referring to FIG. 1, tire components are assembled on a cylindrical drum in the first molding step for manufacturing a conventional tire. The first case thus defined is one in which an inner liner layer, a carcass layer (2), a bead(3), an apex(5) and a sidewall(4) are assembled on the cylindrical drum(1).
FIG. 2(a) illustrates the second molding step, wherein said first case is shaped in a form similar to the completed tire wherein a breaker layer(7) is adhered on said carcass layer and then a tread(6) is adhered on said breaker layer(7). FIG. 2(b) shows that after the second type said breaker layer(7) and tread layer(6) are adhered completely by the pressing of a stiching roller(16) to form a completed green tire.
FIGS. 3, 4 show perspective and sectional views of a tread element (6) cut in regular lengths after extrusion of the tread by the conventional method.
However, as mentioned above, since the tread shaped by a conventional molding process and cut by a knife generally has a low rubber contractile rate and low precision in cutting, its requisite length and cutting cannot readily be obtained in the second molding step.
Due to the above-mentioned reasons, when the tread is assembled in the second step, it is adhered by extending or reducing its length little by little as required depending upon the original length.
Accordingly, the tread sectional profile and its weight distribution along a circumference of the tread are not uniform.
Uneven sectional profile and weight of the tread exert an important effect on uniformity and balance of the final product because the tread has a high weight ratio relative to the whole tire. Furthermore, if adhesive force in the tread is lowered across its section or alien substances penetrate, they cause tire disintegration within a short period of time.
Furthermore, an extruder for manufacturing a tread being utilized in the prior extruding method has a large volume, and requires auxilliary equipment such as facilities for a cooling, cutting, preservation and the like, passing through a conveyer system after extrusion.
Accordingly, it occupies a large space as well as a cost price increase, and maintenance and repair also are difficult.
The present invention solves the defects of the conventional molding method employing the method described above.